Cell culture is a very useful and widely used technique in pharmaceutical development, cell biology, toxicology, bioengineering and tissue engineering fields. Conventional cell cultures are conducted in cell culture vessels, such as in 2, 4, 6, 24, 96 well cell culture plates made from non-degradable polymers such as polystyrene, polypropylene, and polyvinyl chloride, etc. These plates are often been surface treated to improve the hydrophilicity of the surface so that the cells being cultured can better adhere to the 2 dimensional surfaces of the culture plate. In a typical cell culture experiment, cells cultured in the cell culture plastic vessels are grown in cell culture medium in monolayer in a 2-dimensional fashion.
While culturing cells in two dimensions (2D) is a convenient method for preparing, observing and studying cells and their interactions with pharmaceuticals, biological factors and biomaterials in vitro. It does not mimic the cell growth fashion in vivo. In real living body, cells are often growing in three dimensions (3D) and building three dimensional living tissue or organ. Emerging evidence showed that 3D cell culture systems in vitro can facilitate the understanding of structure-function relationship in normal and pathological tissue conditions. Studies also showed that 3D culture is a better model for the cytotoxic evaluation of anticancer drugs in vitro [Harpreet et al, Biomaterials, 2005, 26:979-986]. Moreoevr, growing evidence showed that three-dimensional (3D) environment also reveals fundamental mechanisms of cell function and that 3D culture systems in vitro can facilitate the understanding of structure-function relationship in normal and pathological conditions [Abbott et al, Nature, 2003; 424(6951):870-2; Hutmacher et al, Journal of Biomaterials Science, Polymer Edition, 2001; 12:107-24; Schmeichel et al, Journal of Cell Science, 2003, 116:2377-2388; Zahir et al, Current Opinion In Genetics & Development 2004, 14:71-80; Martin et al, Trends In Biotechnology, 2004; 22:80-6]. It is now well accepted that bone and cartilage-derived cells behave differently in a 3 dimensional (3D) than in a two-dimensional (2D) environment and that the 3D culture systems in vitro are mimicking the in vivo situation more closely than the two-dimensional (2D) cultures (Kale et al, Nature Biotechnology, 2000, 18:954-8; Ferrera et al, Bone 2002, 30:718-25; Tallheden et al, Osteoarthritis and Cartilage, 2004, 12:525-35).
So far the evidence has shown clearly that culturing cells in a 3D environment will offer tremendous advantages over 2D culture environment. However, with the current 3D gel systems, the cultured cells are embedded within the gel matrix which makes the exchange of the nutrient and metabolic products of the cultured cells problematic because of the diffusion limitation of the gel. Also, unlike culturing cells in 2D cell culture plates, in which case cells can be easily detached from the culture plate using a trypsin solution and isolated by centrifugation, cells cultured in 3D gel systems are very difficult to recover or isolate because the cultured cells are embedded within the gel. Additionally, culturing cells within a gel matrix requires preparation of the gel system each time before the culture, which is not only inconvenient to the researchers, especially when the large quantities of cultures have to be done, but also non-consistent between the different batches of gel preparations caused by the slight different ways of gel preparation among different researchers and laboratories.
Polystyrene, polyethylene, polyamide, polyethylene terephthalate), polypropylene and polycarbonate are non-degradable polymers and have been used as a substrate material for conducting two-dimensional (2D) cell culture. Cell culture vessels and membranes made from above mentioned polymers are widely used and commercially available in many different sizes and configurations from many suppliers. Since these polymers are quite familiar to the researchers who are doing cell or tissue culture, it is conceivable that a 3D cell culture system made from these polymers would offer not only the advantages of a 3D culture environment, but also offer many other advantages that a 2D cell culture system would offer, such an well defined surface property and ease of use.
The use of polystyrene in fabrication 3D matrix for cell culture has been little explored. Recently, Baker et al (Baker et al, Biomaterials, 2006; 27, 3136-46) reported that they fabricated a 3D porous fibrous polystyrene matrix using a electro-spinning technique. The obtained fibrous 3D polystyrene matrix was a non-woven mat where the inter-fibrous space served as the porous space. Study data suggested that these polystyrene 3D fibrous scaffolds complemented 2D polystyrene cell culture plate systems. However, the disadvantage of these fibrous polystyrene matrix are The control of the fiber size is difficult; the pore size and shape of the matrix are not well defined; The average pore size was small (˜15 microns), and the fibrous matrix are soft in nature which makes it difficult for further cell culture manipulation without deforming the matrix.
Other researchers also tried to make a more robust porous polystyrene matrix for routine cell culture. They used a high internal phase emulsion (HIDE) as a template to create the porous polystyrene structure (Hayman, et al, J. Biochemical and Biophysical Methods, 2005, 62:231-240). Highly porous polystyrene foams were prepared from poly(styrene/divinylbenzene) system. Study showed that human neurons adhered well to poly-d-lysine coated surfaces and extended neural processes. Neurite outgrowth was particularly enhanced when the surface also received a coating of laminin. However, there are also some disadvantages associated with this polystyrene foams, such as the pore size and pore distribution can not be very well controlled due to the inherent nature of this foaming method, the very torturous porous structure also makes the nutrient exchange difficult.
Due to above mentioned drawbacks associated with the use of current available 3D culture matrix, 2D cell culture is still the primary cell culture method despite the advantages of the 3D culture will offer. Therefore, a 3D culture system which has well defined pore size and porosity for routine 3 dimensional cell culture will be extremely valuable. The present invention provides methods to fabricate 3D cell culture construct which can be used as an insert to the cell culture vessels for conducting 3D cell culture.
Rapid Prototyping (RP) is a technology that produces models and prototype parts from 3D computer-aided design (CAD) model data and model data created from 3D object digitizing systems. Rapid prototyping technologies have also been explored for the development of manufacturing approaches to create surgical implant models for orthopedic and craniofacial surgical procedures. RP systems provide possibilities in fabricating porous 3D object with well controlled channels or pores.
One of the rapid prototyping techniques called 3-Dimensional Printing (3DP) has been used to fabricate bioresorbable porous scaffolds for tissue engineering applications. The technology is based on the printing of an organic solvent binder, such as chloroform and methylene chloride, through a print head nozzle onto a polymer powder bed. However, the removal of entrapped powder is typically quite difficult. Also the bioresorbable aliphatic polyesters can generally only be dissolved in highly toxic solvents such as chloroform and methylene chloride. To date, only bioresorbable scaffolds in combination with particle leaching have been processed by 3DP. In addition, the mechanical properties and accuracy of the specimens manufactured by 3DP still need to be significantly improved.
Other RP technologies, such as stereolithography (SLA) and Selective Laser Sintering (SLS), were used for fabrication of non-bioresorbable polymers scaffolds. SLA is limited by the types of polymers it can use, because it uses only limited photopolymerizable resins supplied from the machine manufactures. And those resins are not suitable for cell culture purposes. SLS, on the other hand, although can use thermoplastic resin particles, bear the same issues as 3DP, i.e. the unbound polymer particles are difficult to remove, and the mechanical properties and the accuracy of the specimens will have to be significantly improved.
Fused Deposition Modeling (FDM) is another manufacturing process that produces 3D objects through the extrusion and deposition of individual layers of thermoplastic materials. It begins with the creation of a conceptual CAD model on the computer. The CAD model is imported into a slicing software (e.g., the QuickSlice™. software offered by Stratasys Inc. of Eden Prairie, Minn.) which mathematically dissects the conceptual 3D model into a series of horizontal layers, followed by the creation of deposition paths within each sliced layer. The tool path data is then downloaded to the FDM machine for scaffold fabrication. The FDM system operates in the X, Y and Z axes. In effect, it draws the designed model one layer at a time. The FDM method involves the melt extrusion of filament polymer materials through a heated nozzle and deposition as thin solid layers on a platform. The nozzle is positioned on the surface of a build platform at the start of fabrication. It is part of the extruder head (FDM head), which also encloses a liquefier to melt the filament material fed through two counter-rotating rollers. Each layer is made of “raster roads” “deposited in the x and y directions. A “fill gap” can be programmed between the roads to provide horizontal channels. Subsequent layers are deposited with the x-y direction of deposition—the “raster angle” programmed to provide different lay-down patterns.
In a FDM system, the thermoplastic polymer used in fabricating the part has to be in a filament form in order for the thermoplastic polymer material to feed into the temperature-controlled FDM extrusion head, where it is heated to a semi-liquid state. The head extrudes and deposits the material in thin layers onto a fixtureless base. The head directs the material into place with precision. The material solidifies, laminating to the preceding layer. Parts are fabricated in layers, where each layer is built by extruding a small bead of material, or road, in a particular lay-down pattern, such that the layer is covered with the adjacent roads. After a layer is completed, the height of the extrusion head is increased and the subsequent layers are built to construct the part. The layer by layer fabrication allows design of a pore structure which varies across the scaffold structure. So far, only a few non-resorbable polymeric materials, such as polyimide, ABS, and polycarbonate are available from the FDM manufacturer Stratasys Inc for se in the FDM RP systems. If a different polymer materials need to be used in the FDM RP system, the material has to be pre-manufactured into a filament form which, although can be done by melt extrusion, will add an extra step to the manufacturing process. Also, the FDM process involves two hot melting process, i.e making filament by melt extrusion and FDM melt deposition. It is well known that repeated heating will cause polymer to degrade and produce leachable smaller molecules, which is not a problem in making a model for industrial use, but will be a significant issue for cell culture use. Therefore, FDM process is not suitable for making porous 3D polymer constructs for cell culture use.
Therefore, there is a need to have a RP system which can directly use thermal plastic polymer particles/pellets without first making polymer filament. An RP system which has a hot melt extrusion system will be able to directly deposit polymer material in a filament form and therefore minimizes the possibility of heat induced degradation. This system would be particularly suitable for making 3D construct for cell culture use.